The signal generation circuitry for mobile radios is a complex process with component cost and component count issues being of high priority in the mobile radio design. One typical arrangement to generate frequencies in mobile radio units is to use voltage controlled oscillator (VCO) designs. Frequency modulation circuits require VCO output signals to be modulated with a defined modulation deviation in order to transmit a given amount of information. To meet specification requirements this deviation may not exceed lower or upper limits over a specified frequency range and temperature variation. As there are a number of complicated effects that have an impact on the modulation deviation performance, a trade off between expected stability and expensive specific components has to be found. This is particularly the case with field effect transistor (FET) oscillators where such trade offs limit the performance in wideband VCO designs, principally over temperature variation. Hence, there is a need to fully compensate for this temperature dependency in the frequency generation design.
A typical prior art frequency generator circuit 10, often termed a frequency resonator tank circuitry is shown in FIG. 1. The frequency resonation is formed by an inductance 26, a voltage variable capacitance 24 and further inductive and capacitive elements. A FET 40 is coupled to the frequency resonator tank circuit via a gate capacitance 28. A capacitive Colpitts divider 32, 34 between the gate-source port and the source-ground port of the FET 40--"Cgs" and "Csg"--adds positive feedback for oscillation generation. An inductive choke 38 isolates the RF signal at source in the DC path from ground. A diode 30 rectifies the peak RF amplitude to generate a negative gate voltage which forms an automatic gain control function (AGC) for the feedback amplification to set the oscillation amplitude to a defined, and not clamped level. The RF output signal is available at the source port 46.
To modulate the frequency being generated by the frequency resonator tank circuit, a modulation signal 11 is routed via resistor 12 to inductor 14, capacitor 16 and modulation varactor 18. Dependent upon the impedance of the modulation capacitor 20, modulation amplitude and component characteristics, the circuit represents a modulation dependent impedance and provides a modulation impedance signal 22 to the resonator tank circuit. The modulation circuit is connected to port A, for best coupling to the resonator tank circuitry or to the FET source port 46 for reduced modulation coupling.
As previously highlighted, a major disadvantage with optimising such VCO circuitry is with the varying modulation deviation sensitivity with respect to varying temperature. Typically, the modulation-temperature performance requires a significant amount of measurements to be taken of both the modulation and resonator tank circuits. Consequently, optimisation of such circuits with regard to temperature variation is often neglected resulting in over or under compensation of the modulation sensitivity of the frequency generation circuits.
Furthermore, in known designs, the modulation capacitance 20 coupling the modulation circuit to the resonator tank circuit is chosen as a special component with suitable temperature factor, for example a Murata N750 type capacitor (750 parts per million capacitance variation if the circuit indicates a positive temperature characteristic), to stabilise the VCO performance. This requires a unique component and consequently an extra reel in any automatic production process. Additionally it is often a critical aspect of the VCO design to use a specific highly specified component with the desired temperature compensation factor, to avoid the performance being degraded.
This invention seeks to provide a voltage controlled oscillator circuit, and method of optimisation for such a voltage controlled oscillator circuit, to mitigate at least some of the problems highlighted above.